1. Field of the Invention
The present invention relates to a sampling system for engine exhaust gas analysis apparatus and, more particularly, to such a system which eliminates the condensation of moisture on the sensitive area of an oxygen sensor used therein.
2. Description of the Prior Art
It has been found that an oxygen analyzer can be combined with an HC/CO analyzer to provide significant information when analyzing the exhaust gas of a motor vehicle for engine diagnostic purposes. Excess oxygen in the exhaust gas of motor vehicles with catalytic converters has been identified as a major cause of increased sulfate emissions. The presence of increasing levels (spiking) of oxygen in the exhaust gas of motor vehicles is often the result of intermittent misfire or "lean roll." This condition can be detected by sampling a portion of the exhaust gas from the automotive vehicle and conducting the sample to an oxygen sensor. By coupling the sensor with means for differentiating the signal output thereof, both the quantity and rate of change of oxygen in the exhaust gas is available. Apparatus can then be provided for indicating and/or alarming quantity and rate of change values outside preset limits. A technician can then use this information to adjust the idle mixture of an automobile engine to prevent a lean roll.
Engine exhaust gas analysis apparatus including an oxygen sensor and means for sampling a portion of the exhaust gas and conducting the sample to the sensor is disclosed in U.S. Pat. No. 4,030,349, assigned to the assignee of the present application. In such patent, an oxygen sensor connected to a probe is inserted into the exhaust pipe of an automobile to conduct a portion of the exhaust gas to the sensor. The oxygen sensor generally comprises a pair of electrodes mounted within a body and electrically connected by an electrolyte and separated from the exhaust gas sample by means of a membrane that is permeable to oxygen, but impermeable to the electrolyte.
As the exhaust gas sample is brought into contact with the membrane, oxygen diffuses through the membrane to contact one of the electrodes in the presence of the electrolyte. A current flow results which is linear with the partial pressure of oxygen being sampled. Thus, the current can be measured and correlated to the amount of oxygen in the sample. Such sensors are quite rapid, simple in operation, and especially suited for determining either gaseous or dissolved oxygen in liquids or gases.
The exhaust gas from a motor vehicle is quite hot, typically in the range of 350.degree.-500.degree. F. In addition, such exhaust gas is relatively high in humidity, typically containing 12-16% H.sub.2 O. When the hot exhaust gas impinges on the cold oxygen sensor, the moisture therein condenses, often coating the sensitive area of the sensor. This has been found to mask the changes in the oxygen concentration in the sample, substantially reducing the effectiveness of such exhaust gas analysis apparatus.